This invention relates generally to surface acoustic wave devices and more particularly to surface acoustic wave devices having reduced vibration sensitivity.
As is known in the art, surface acoustic wave devices (SAW Devices) such as resonators and delay lines are used a variety of applications. Generally, a SAW device includes a pair of transducers, each having a set of conductive members disposed on or recessed within a surface which supports surface wave propagation. Typically, the surface which supports surface wave propagation is comprised of a piezoelectric crystallographic material. In many applications for SAW devices, it is important to provide a package having a relatively small size which is properly mounted to reduce the so-called vibration sensitivity of the SAW device. Vibration sensitivity or microphonics refers to the frequency changes or noise resulting from mechanical stresses applied to the SAW device. Reduction in vibration sensitivity is particularly important when the SAW device is used to stabilize the frequency characteristic of an oscillator.
It is known that the resonant frequency of a SAW stabilized oscillator is sensitive to external vibration or changes external stresses applied to the SAW device. This sensitivity results from the external stresses on a piezoelectric causing changes in surface wave velocity characteristic of the device.
Generally, one of the more difficult problems encountered in using SAW devices is to reduce the effects of thermal expansion and proper mounting of the SAW device to reduce vibration sensitivity. It is known in the art to use RTV rubbers or natural rubbers between the SAW device and a mounting surface to accommodate differences in thermal expansion between the SAW device and the mounting surface. Artisans have also attempted to use RTV rubbers or natural rubbers reduce vibration sensitivity. When such soft stress damping material is uniformly attached to the bottom of the SAW device it was thought by the art that the stress damping material would act to damp out vibrations provided to the device.
Nevertheless, vibration sensitivity of SAW devices has limited their usefulness to low vibration environments for applications such as frequency stabilizing elements in high precision oscillators, because vibration sensitivity can substantially degrade the phase noise spectrum of a SAW oscillator when subjected to vibration. A reduction in SAW device vibration sensitivity by one order of magnitude, however, would reduce the corresponding contribution to phase noise by two orders of magnitude. A two order of magnitude improvement in phase noise levels would permit SAW devices to readily compete in electrical characteristics with bulk acoustic wave (SAW) devices in applications where the vibration environment is severe.
Airborne applications are one environment in which vibration sensitivity is a problem. In airborne applications, there are two types of acceleration which may be encountered. The first is a DC type of vibration resulting from acceleration of the device during flight of the airborne vehicle. DC vibration will cause a uni-directional deformation in the substrate of the SAW device during constant acceleration and a corresponding shift in the frequency characteristic of the device. The second is an AC type of vibration resulting from stresses, which vary in direction and magnitude. AC vibration will cause corresponding bi-directional deformation of the crystal and concomitant fluctuations in the frequency characteristics of the SAW device.